This invention relates to a superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called "MRI"), and more particularly to an improved and simplified passive shimming arrangements in such an assembly.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that when a power source is initially connected to the coil for a short period of time to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of resistance, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of MRI.
In order to compensate for the inhomogeneities in MRI magnets, various arrangements including correction coils and passive ferromagnetic shim materials have been used.
Open architecture MRI magnets tend to produce increased field inhomogeneity due to increased coil deformation and coil misalignment. Prior art superconducting magnet designs are directed at minimizing such inhomogeneity during the design stage, and then add passive shim systems to reduce the inhomogeneity that remains after the manufacturing cycle due to manufacturing tolerances and design restrictions.
The shimming frequently utilizes a combination of correction coils and passive ferromagnetic shims with the superconducting magnet adjusted at the factory to provide a homogeneous magnetic field in the imaging bore of the magnet. Since the passive shims are positioned between the warm imaging bore and the gradient coil it is difficult with current designs to access and adjust or change the passive shims after the gradient coil is installed and while minimizing the profile or space occupied by the shim assembly. However, due to magnetic material in the vicinity of the magnet at the installation site, it is frequently necessary to reshim the magnet to provide the required field homogeneity.
Because of the difficulty in adjusting the configuration of the passive shims, the on-site adjustment has frequently been to vary the current through the correction coils. Since the correction coils associated circuitry and leads are extremely expensive a magnet which minimizes or eliminates the need for such a correction coil assembly is highly desirable.
Accordingly, it is highly desirable to be able to do the rough shimming at the on-site and the fine shimming at the installation site without disturbing the gradient coils while providing the ability to readily access particular shim members without disturbing other shim members.